Implementation of Multi-Access Tag Based on OFDM ▪ Hengzhe Ding 5140309057
Outline ▪ Overview ▪ Background Information ▪ Basic Principle ▪ Hardware Implementation ▪ Test Result ▪ Follow-up Work ▪ Conclusion ▪ Reference ▪ Q & A
Overview Traditional Wi-Fi routers have been considered power-consuming device thus cannot be used in the field of IOT. In order to achieve the low-power feature required in IOT, power hungry RF component and low-power consuming digital components of traditional Wi-Fi transmitter are separated. This semester I built a prototype hardware.
Background Information ▪ OFDM DM OFDM DM stands for Orthog ogonal nal Frequenc equency y Division ision Multipl tiplex exing ing, which is a modulating technique used in Wi-Fi signal transmission. OFDM signal is multi-carrier modulated signal, which means it has multiple subcarriers to transmit the signal. Take Wi-Fi transmission as an example, in each channel of the total three channel of Wi-Fi, there are 52 subcarriers, thus the transmitted signal x(t) can be expressed as follows, 𝑜=26 𝑜=26 𝑌 𝑙 𝑓 𝑘2𝜌 𝑔 𝑑 +𝑙𝑔 0 𝑢 𝑦 𝑢 = 𝑦 𝑙 𝑢 = 𝑜=−26 𝑜=−26 Where 𝑌 𝑙 is the symbol transmitted on 𝑙 𝑢ℎ subcarrier, 𝑔 𝑑 = 2.4𝐻𝐼𝑨 and 𝑔 0 = 312.5𝐿𝐼𝑨 in the case of Wi-Fi transmission
Background Information ▪ OFDM DM Note that 𝑌 0 = 0 and 𝑦 −21 (𝑢), 𝑦 −7 (𝑢), 𝑦 7 (𝑢), 𝑦 21 (𝑢) are served as pilot signal used for synchronization and contain no data.
Background Information ▪ Backs ckscat catter ter Unlike traditional Wi-Fi transmitter, instead of generating RF signal directly, tags in our system modulate the radar cross-section of the antenna to change the reflected signal. This is achieved by switching the impedance of the circuits followed by the antenna, when changing such impedance, the reflect coefficient is altered as well. The backscattered signal can be expressed as follows 𝑇 𝑐𝑏𝑑𝑙𝑡𝑑𝑏𝑢𝑢𝑓𝑠 = Г · 𝑇 𝑗𝑜𝑑𝑗𝑒𝑓𝑜𝑢 𝑎 𝑏 − 𝑎 𝑑 where Γ = 𝑎 𝑏 + 𝑎 𝑑 , 𝑎 𝑏 , 𝑎 𝑑 is the impedance of the antenna and the impedance of the circuit followed by the antenna, in most cases, 𝑎 𝑏 = 50Ω .
Basic Principle The process of our system is first to generate a single-tone RF signal in the transmitter side, then shift the frequency to the desired 48 frequencies of subcarriers that contain data and modulate data we want to transmit on shifted signals. Therefore, there are two function of tags in our system, frequ quency ency shiftin fting g and modula lati ting ng.
Basic Principle ▪ How do we shift ft the single-to tone ne freque uency y to desired ed 48 freque uenc ncies ies? We need to use two equations show as follows 2𝑡𝑗𝑜2𝜌𝑔 𝑑 𝑢 ⋅ 𝑡𝑗𝑜2𝜌Δ𝑔𝑢 = cos 2𝜌 𝑔 𝑑 − Δ𝑔 𝑢 − cos 2𝜌 𝑔 𝑑 + Δ𝑔 𝑢 ∞ 𝑇𝑟𝑣𝑏𝑠𝑓 Δ𝑔𝑢 = 4 1 𝑜 𝑡𝑗𝑜2𝜌𝑜Δ𝑔𝑢 𝜌 𝑜=1,3,5… If we suppose 𝑇 𝑗𝑜𝑑𝑗𝑒𝑓𝑜𝑢 = sin 2𝜌𝑔 𝑑 𝑢 and replace Γ in previous slide with 𝑇𝑟𝑣𝑏𝑠𝑓 Δ𝑔𝑢 , according to the trigonometry equation shown above, we obtain 𝑇 𝑐𝑏𝑑𝑙𝑡𝑑𝑏𝑢𝑢𝑓𝑠 = 𝑇𝑟𝑣𝑏𝑠𝑓 Δ𝑔𝑢 ⋅ 𝑡𝑗𝑜2𝜌𝑔 𝑑 = 𝜌 4 𝑑𝑝𝑡2𝜌 𝑔 𝑑 − Δ𝑔 𝑢 − cos 2𝜌 𝑔 𝑑 + Δ𝑔 𝑢 ∞ + 𝜌 [𝑑𝑝𝑡2𝜌 𝑔 𝑑 − 𝑜 ⋅ Δ𝑔 𝑢 − 𝑑𝑝𝑡2𝜌 𝑔 𝑑 + 𝑜 ⋅ Δ𝑔 𝑢] 4 𝑜=3,5…
Basic Principles ▪ How w do we shift ft the single gle-to tone ne freque quency ncy to desi sired red 48 freque quencies? ncies? The period toggle (frequency Δ𝑔 ) between +1 and -1 of reflective coefficient Γ can be realized by a RF switch which toggle between the state of short circuit and open circuit. Γ = +1
Basic Principles ▪ How w do we shift ft the single gle-to tone ne freque quency ncy to desi sired red 48 freque quencies? ncies? The period toggle (frequency Δ𝑔 ) between +1 and -1 of reflective coefficient Γ can be realized by a RF switch which toggle between the state of short circuit and open circuit. Γ = −1
Basic Principles ▪ How w to modula late te? We only want to implement the simplest case of digital modulation - BPSK in our system. The two phases required in BPSK is 0 (data 1) and 𝜌 (data 0), which means the desired signal is cos2𝜌(𝑔 − ∆𝑔)𝑢 − cos2𝜌(𝑔 + ∆𝑔)𝑢 and − (cos2𝜌 𝑔 − ∆𝑔 𝑢 − cos2𝜌 𝑔 + ∆𝑔 𝑢) respectively. Accordingly, the square waves corresponding to the two different phases are 𝑇𝑟𝑣𝑏𝑠𝑓 Δ𝑔𝑢 (data 0) and −𝑇𝑟𝑣𝑏𝑠𝑓(Δ𝑔𝑢) (data 1). 𝑇𝑟𝑣𝑏𝑠𝑓 Δ𝑔𝑢 Output to the MUX C following stages −𝑇𝑟𝑣𝑏𝑠𝑓 Δ𝑔𝑢 Data (0 or 1)
Basic Principles ▪ How w to solve ve the prob oblem em of spectrum ctrum wast ste? e? In the diagram, 𝜐 0 is used to create phase 𝜌 in BPSK modulation and 𝜐 1 is 4 1 used to change the expression of square wave from 𝑜 𝑡𝑗𝑜2𝜌𝑜Δ𝑔𝑢 to 𝜌 ∑ 4 1 𝜌 ∑ 𝑜 𝑑𝑝𝑡2𝜌𝑜Δ𝑔𝑢 .
Basic Principles ▪ How w to solve ve the prob oblem em of spectrum ctrum wast ste? e? The incoming signal: 𝑇 𝑗𝑜𝑑𝑗𝑒𝑓𝑜𝑢 (𝑢) = 2𝐵𝑡𝑗𝑜2𝜌𝑔 𝑑 𝑢 Output signal of RF splitter: 𝑠 1 (𝑢) = 𝑠 2 (𝑢) = 𝐵𝑡𝑗𝑜2𝜌𝑔 𝑑 𝑢 𝑑 𝑢 + 𝜌 1 𝑢 ∠45 ◦ = 𝐵𝑡𝑗𝑜 2𝜌𝑔 𝑠 4 , 𝑠 2 𝑢 = 𝐵𝑡𝑗𝑜2𝜌𝑔 𝑑 𝑢; 1 𝑢 ∠45 ◦ · 4 𝜌 ∑ 1 𝑠 1𝑐𝑏𝑑𝑙𝑡𝑑𝑏𝑢𝑢𝑓𝑠 𝑢 = 𝑠 𝑜 𝑡𝑗𝑜2𝜌𝑜𝛦𝑔𝑢 2𝐵 𝜌 2𝐵 𝜌 ≈ − 𝜌 𝑑𝑝𝑡[2𝜌 𝑔 𝑑 + 𝛦𝑔 𝑢 + 4 ] + 𝜌 𝑑𝑝𝑡[2𝜌 𝑔 𝑑 − 𝛦𝑔 𝑢 + 4 ] 2 𝑢 · 4 𝜌 ∑ 1 𝑠 2𝑐𝑏𝑑𝑙𝑡𝑑𝑏𝑢𝑢𝑓𝑠 𝑢 = 𝑠 𝑜 𝑑𝑝𝑡2𝜌𝑜𝛦𝑔 2𝐵 2𝐵 ≈ 𝜌 𝑡𝑗𝑜[2𝜌 𝑔 𝑑 + 𝛦𝑔 𝑢] + 𝜌 𝑡𝑗𝑜[2𝜌 𝑔 𝑑 − 𝛦𝑔 𝑢]
Basic Principles ▪ How w to solve ve the prob oblem em of spectrum ctrum wast ste? e? 1𝑐𝑏𝑑𝑙𝑡𝑑𝑏𝑢𝑢𝑓𝑠 (𝑢)∠45 ◦ + 𝑠 𝑇 𝑐𝑏𝑑𝑙𝑡𝑑𝑏𝑢𝑢𝑓𝑠 (𝑢) = 𝑠 2𝑐𝑏𝑑𝑙𝑡𝑑𝑏𝑢𝑢𝑓𝑠 (𝑢) = 4𝐵 𝜌 𝑡𝑗𝑜2𝜌(𝑔 𝑑 + ∆𝑔)𝑢 As you can see from the final result, there only exists frequency component 𝑑 + ∆𝑔) and the ’mirror’ component, (𝑔 𝑑 −Δ𝑔) , is eliminated. (𝑔
Hardware Implementation According to the diagram, the devices we need to build the hardware of the tag are: one RF splitter/combiner, two RF phase shifter(one create 45 ◦ and the other create 0 ◦ , considering the insertion loss caused by the device, I chose two of such device to make sure the amplitude of output of the two phase shifter are approximately the same), two SPDT/SPST switch and the list of device I choose are shown as follows. Devic ices es Models ls RF splitter/combiner BP2U+ RF phase shifter JSPHS2484+ RF switch ADG902
Hardware Implementation The clock management part, which is circled red in previous diagram, is written in VHDL and implemented on Nexys 3 FPGA board. The prototype I built is shown below.
Test Result ▪ Test t Enviro ronment ent - Lab 4-103, SEIEE Building ▪ Test t Instru trument ment - Spectrum analyzer and RF signal generator ▪ Test t Date te - May 11, 2017 ▪ Test t Paramete rameters rs - 𝑔 𝑑 = 2.4𝐻𝐼𝑨, ∆𝑔 = 20.3125𝑁𝐼𝑨, 𝑄 𝑢𝑠𝑏𝑜𝑡𝑛𝑗𝑢 = 20𝑒𝐶𝑛 ▪ Test t Distanc tance - Very close(less than 10cm) ▪ Test t Result ults - 𝑔 𝑐𝑏𝑑𝑙𝑡𝑑𝑏𝑢𝑢𝑓𝑠 ≈ 2.42𝐻𝐼𝑨 , 𝑄 𝑠𝑓𝑑𝑓𝑗𝑤𝑓𝑒 = −8𝑒𝐶𝑛 𝑄 𝑐𝑏𝑑𝑙𝑡𝑑𝑏𝑢𝑢𝑓𝑠 = −45𝑒𝐶𝑛
Test Result ▪ The test phenomenon is shown as follows
Follow-up Work ▪ Increase the power of the backscattered signal and SNR ▪ Further test the function of modulation ▪ Combined test with other parts of our system
Conclusion ▪ Implement a prototype of low-power, multi-access tag based on OFDM. The feature of low-power is achieved since we have removed the power hungry RF component. ▪ I take the hardware diagram of [2] as a reference and design our own tag. The tag can serve the function of frequency shifting and the problem of spectrum waste caused by ’mirror’ component generated in the process of frequency shifting is solved as well. ▪ The other function, modulating remain tested.
Reference [1] Bryce Kellogg, Vamsi Talla, Shyamnath Gollakota, and Joshua R Smith. Passive wi-fi: Bringing low power to wi-fi transmissions. 2016. [2] Pengyu Zhang, Dinesh Bharadia, Kiran Joshi, and Sachin Katti. Hitchhike: Practical backscatter using commodity wifi. In ACM SENSYS, 2016.
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